1. Field of the Invention
The present invention relates to an image forming apparatus having a transfer charger for transferring an image formed on an image bearing member onto a transfer material, and an electricity removing member for removing electricity from the transfer material in order to separate the transfer material from the image bearing member.
The image forming apparatus may be of electrophotographic type or of electrostatic type.
2. Related Background Art
In the past, there have been proposed image forming apparatuses in which a toner image formed on a photosensitive drum (image bearing member) is transferred onto a transfer material by a transfer roller to which voltage is applied and electricity is removed from the transfer material by an earthed electricity removing needle (electricity removing member) in order to separate the transfer material from the photosensitive drum. After the transferring operation, residual toner remaining on the photosensitive drum is removed by a cleaning device, and the toner image transferred to the transfer material is fixed to the transfer material by a fixing device.
However, depending upon the kinds of the transfer materials and/or an environmental condition, when the transfer material is separated from the photosensitive drum, the transfer material often cannot be separated from the photosensitive drum effectively.
On the other hand, when the transfer material is separated from the photosensitive drum, if a trail end of the transfer material is charged with the same polarity as that of the transfer voltage by peel charge, the separation of the trail end of the transfer material from the photosensitive drum will be often delayed. If separation of the trail end of the transfer material from the photosensitive drum is delayed, the trail end of the transfer material will be pulled toward a direction to which the drum is rotated. Consequently, due to "transfer material trail end splash" having the tendency of splashing the trail end of the transfer material toward the drum, the trail end of the transfer material is contacted with a bottom of a cleaning container of the cleaning device to distort the non-fixed toner image on the transfer material or to smudge the trail end of the transfer material by the toner scattered and adhered to the bottom of the cleaning container. Particularly, when the above-mentioned transfer roller is used as a transfer material, since the transfer material is closely contacted with the transfer roller, the above problem becomes more serious.
In order to eliminate the poor separation problem, if the electricity is forcibly removed from the transfer material by applying great voltage having the same charging polarity as that of the toner image to the electricity removing needle, the potential of the transfer material will be decreased, with the result that an electrostatic force for holding the non-fixed toner image on the transfer material is weakened. Consequently, when the transfer material holding the non-fixed toner image thereon is sent to the fixing device, since so-called "fixing offset" occurs (i.e., the toner image on the transfer material is electrostatically adhered to a fixing roller), the image is distorted and/or smudged.
Further, when the great voltage having the same charging polarity as that of the toner image is applied to the electricity removing needle, if moisture is absorbed to the transfer material under the high temperature/high humidity environment, a so-called "transfer void" problem will occur. The transfer void is poor transfer caused when the transfer charge to be applied from the transfer roller to the transfer material to effect the transferring of the toner image cannot be held on the transfer material due to the reduction of resistance of the transfer material to escape to the electricity removing needle.